1. Field of the Invention
This invention relates to an improvement in polymeric colorants. More particularly, it involves polymeric colorants with improved solubility at acidic pH's and an improved method for their achievement.
2. The Prior Art
U.S. Pat. No. 3,920,855 of Dawson, et al., issued Nov. 18, 1975, and U.S. Pat. No. 4,018,826 of Gless, et al. issued Apr. 17, 1977, disclose that polymeric colorants can offer attractive properties such as inability to be absorbed through the walls of the gastrointestinal tract. This nonabsorption means much reduced and possibly eliminated risks of systemic toxicity and suggests advantageous application of stable polymeric colorants in edibles such as foods, beverages and the like. It is a common characteristic of such applications to have aqueous substrates, or to at least contain an aqueous phase, in which the colorant is to be dissolved. These applications also often present acidic environments of use as the majority of food and beverage applications are acidic. Thus, an important property of a polymeric colorant is often its ability to dissolve or remain in solution in an acidic aqueous environment.
One way to achieve this desired acid solubility is to incorporate in the polymeric colorant anionic groups such as carboxyl groups, sulfonate groups or phosphonate groups.
Now, a polymeric colorant of the type disclosed by Dawson, et al., and Gless, et al., is of two parts--a non-chromophoric often alkyl backbone and a plurality of optical chromophores covalently affixed thereto. These anionic groups can be present either attached to the backbone or as part of the chromophores. In either positive they serve to impart water solubility to the final polymeric colorant product.
In the Dawson, et al., and the Gless, et al., disclosures, the chromophore units are covalently bound to the backbone through sulfonamide or amine linkages. In situations where there is very complete substitution of these amine groups by sulfonate groups or by sulfonate-containing chromophores, the character of the overall polymeric colorant is anionic and solubility in acidic aqueous enviroments is fully adequate. In situations where there is a substantial proportion of unsubstituted primary alkyl amines or dialkyl amines present in the polymeric colorant, as can occur either with low degrees of amine substitution by aromatic chromophore units or with substitution by chromophores through an alkyl link, it is seen that the polymeric colorants often haze and precipitate from solution at acidic pH's. In light of the success of the present invention it now appears that at basic to neutral pH's such colorants have a distinctly anionic character imparted by their COO.sup.-, SO.sub.3.sup.- or PO.sub.3.sup.= groups and are soluble. It now appears that at acidic pH's the amines begin to become protonated and the polymer approaches an isoelectric state. In view of the present teachings one may explain this precipitation as that at their isoelectric point the polymeric colorants form insoluble intramolecular and cross-linked salts. This isoelectric condition generally occurs at pH's of 2-4 for amine-containing polymeric colorants. This is an unfortunate pH to have precipitation occur as it is the pH of many soft drinks and most fruit-flavored foods--major applications for food colors.
U.S. Pat. No. 4,169,203 of Wingard and Dawson, which matured from application Ser. No. 743,205, filed concurrently with our parent Ser. No. 743,203 discloses that this troublesome hazing and precipitation can be eliminated by acetylating a substantial proportion of the residual amine groups of such polymeric colorants thereby converting the amines to amides.
Acetylation has been applied to crude mixtures outside the area of colorants, for example, in biological systems. In these prior art uses, to our knowledge, large excesses of acetic anhydride, such as 20-50-fold excesses, have been necessary to achieve full acetylation.